The invention relates to hydraulic control arrangements. More particularly, this invention concerns a control apparatus for a hydraulic power consumer, particularly in mobile agricultural machines.
Conventionally (see for example German Offenlegungsschrift No. 22 32 857) such an arrangement includes a reversing valve actuated by a magnetic valve and operative to supply pressure medium to a consumer. The reversing valve is provided with a first relief valve for regulating pressure medium flow in this valve. The reversing valve is further provided with a piston which has a precise control chamfer. The piston bounds in this valve a control chamber which is operatively connected with the magnetic valve, through another relief valve. The other relief valve is operative to damping the pressure oscillations in the user resulted when the consumer or user moves from a stationary position.
A shortcoming of such a construction resides in the fact that the desirable damping function of this arrangement occurs too late; i.e., the relief valve reduces the pressure medium flow only when the pressure in an inlet chamber of this arrangement (resulting from the force of a spring which actuates the piston) exceeds that which is exercised on the user from an outside load. In such a case the piston is urged to move into its end position, so that the precise control chamfer cannot operate efficiently with regard to dampening the user's oscillations.
Another shortcoming of such a construction resides in providing two different valves which are necessary to alternately supply and withdraw the pressure medium to and from the user, respectively. That results in relatively high expenses and makes the possibility of leakage in such a system very likely.
Furthermore, the damping means in such a construction have to be installed immediately on the valve which operates for lowering the user. This fact leads to a situation where, if the load on the user is small, the lowering action can be carried out only slowly, which is obviously disadvantageous for the productivity of such an arrangement.
Also, it is to be understood that leakage of the pressure medium when the user is in its stationary position or is being lowered, leads to very undesirable consequences.
Utilization of two different servovalves raises the expense of such an arrangement. Besides that, the provision of the second valve for controlling the operation of the user creates a danger of malfunction of the user.